The present invention relates to an oil condition sensor used to check the degree of contamination of automotive transmission oil or engine oil.
Such an oil condition sensor is mounted in an oil container for automotive transmission oil or engine oil and is used to check the degree of contamination of the oil with iron powder or other metallic powder that are produced when automotive parts are abraded against each other. A typical such sensor is disclosed in JP patent publication 2002-286697A (see especially its FIGS. 1–4). It includes a rod adapted to be immersed in oil. The rod carries magnets at its tip. A pair of electrodes are provided around the magnets, axially spaced from and opposed to each other. The sensor produces a signal corresponding to the resistance in the oil between the electrodes, which in turn corresponds to the amount of metallic powder in the oil. At least one of the pair of electrodes comprises a resistor. By detecting the resistance between the electrodes which changes with the area of the end face of the resistor covered by iron powder that is magnetically attracted toward the outer surface of the magnets and stuck on the end face of the resistor, it is possible to detect the amount of iron powder in the oil.
In this publication, it is necessary to use a resistor as one of the pair of electrodes or on the outer periphery of an insulating cover provided on the inner periphery of the electrodes. Such a resistor tends to be internally stressed when the load and/or temperature changes. Such internal stresses in turn change the carbon sequence, thereby causing fluctuations in the specific resistance of the resistor. Because the specific resistance thus easily fluctuates, it is impossible to stably and reliably detect the resistance between the electrodes, so that no accurate determination of the degree of contamination of oil is possible.
In order to a solve this problem, the inventor of the present invention proposed an oil condition sensor in (JP patent application 2004-91029) which uses no resistors, of which the specific resistance tends to fluctuate according to the load applied and the temperature, and thus can stably check the degree of contamination of oil. This oil condition sensor includes a pair of electrodes with one of the electrodes comprising a plurality of conductors each arranged so as to oppose the other electrode. By counting the number of conductors that are electrically connected to the other electrode through metallic powder in the oil, it is possible to stably detect the amount of metallic powder in the oil. In order to count the number of conductors that are electrically connected to the other electrode, the conductors are connected to a power supply through separate fixed resistors or other members that react to electricity.
The inventor of the present invention also proposed in JP patent application 2004-148984 an oil condition sensor including conductors similar to those disclosed in JP patent application 2004-91029 which are connected to a power supply through separate resistors having different resistances from each other. By counting the number of conductors that are electrically connected to the other opposed electrode, it is possible to detect the amount of metallic powder in the oil. The resistances of the resistors are determined such that the number of conductors that are electrically connected to the other opposed electrode will be in a linear relationship with the sensor output.
In either of the oil condition sensors, since metallic powder stuck on the electrodes is in oil, of which the temperature fluctuates, when the oil temperature rises and its viscosity decreases, metallic powder particles tend to more closely contact each other. The electrical resistance between the electrodes thus decreases. Conversely, when the oil temperature falls and its viscosity increases, metallic powder particles tend to less closely contact each other, thus increasing the electrical resistance between the electrodes.
Thus, even if the amount of metallic powder stuck on the electrodes is unchanged, the number of conductors that are electrically connected to the opposed electrode or the sensor output may change if the oil temperature changes. In particular, if the oil temperature falls, even if the amount of metallic powder stuck on the electrodes is unchanged or increases, the number of conductors electrically connected to the opposed electrode or the sensor output may fall.
An object of the present invention is to provide an oil condition sensor which can stably check the degree of contamination of oil even if the oil temperature changes.